height, and a and I are the sides of the squares forming the top
and the bottom surfaces.
The truncated pyramid was a familiar sight during the con-
struction of a pyramid and, no doubt, calculations had to be
FIG. a. Rhind Mathematical Papyrus (Brit. Mus. PL XIV). -Problems
dealing with rectangles, circles, and triangles.
made in connexion with questions of labour and material. An
obelisk is a small pyramid on top of a truncated pyramid.
It is possible that the result was arrived at by experiments
with models. For the purpose, Nile mud in a half-dried, semi-
plastic state would serve as well as the modern plasticine. The